The invention relates to the mass spectrometric analysis of surface material by means of laser ablation.
The mass spectrometric analysis of material on or in surfaces of solid bodies has many applications, ranging from imaging mass spectrometry of substance distributions in thin tissue sections or thin-layer chromatographic plates to the analysis of arbitrarily applied analytical samples on sample supports. There are many different methods for removing the surface material, some of which also ionize immediately, for example vacuum RF sparks (SSMS=“spark source mass spectrometry”), sputtering (SIMS=“secondary ion mass spectrometry”) or matrix-assisted laser desorption and ionization (MALDI). Other removal methods such as laser ablation only remove neutral molecules of the material being analyzed. The neutral molecules must then be ionized, for example in an inductively coupled plasma (ICP) in which the molecules are broken down into ionized atoms, thus making it possible to determine the elementary composition of the surface material. The method is used to analyze metal atoms in organic material, for example.
In recent years, the focus of attention has been on imaging mass spectrometry (IMS); and particularly matrix-assisted laser desorption (MALDI) is used as the method of removal and ionization. This requires matrix substance to be applied to the sample surface, however. The method, which is particularly successful for thin tissue sections, requires that a relatively thick, very uniform layer of matrix material is applied, by spraying as a solution in individual layers, in order to hold back contaminating substances and transport proteins to the surface. A favorable method for this has been described in the Patent Application DE 10 2006 019 530.2.
This method has the disadvantage, however, that the layer of matrix substance applied limits the lateral spatial resolution to some 200 micrometers at best, even if finer focusing of the desorbing laser beam is achieved. It is therefore not possible to look into individual biological cells and determine the composition of individual organelles, for example.
Other applications of mass spectrometry have already shown that electric near fields, which can be generated in front of the probes of an atomic force microscope (AFM), for example, can be used to produce a near-field focusing of laser beams, thus allowing vaporization pits down to a mere 30 nanometers or so in diameter to be produced. This lateral spatial resolution of around 30 nanometers makes it possible to vaporize a volume with around 10,000 molecules if one assumes an average molecular weight of around 1,000 Daltons. With 100 nanometer vaporization pits, it is possible to vaporize some 300,000 molecules. Such numbers of molecules are very low for mass spectrometric analyses; they require highly effective methods of ionization and ion transfer to the mass analyzer.